`RBr` when treated with `AgCN` in a highly polar solvent gives `RNC` whereas when it is treated with `NaCN` it gives `RCN`. Explain.

Answer

To understand the different products formed when RBr is treated with AgCN and NaCN, let's break down the reactions step by step:

### Step 1: Understanding the Reactants
- **RBr**: This is an alkyl bromide, where R represents an alkyl group and Br is a bromine atom.
- **AgCN**: Silver cyanide, which is predominantly covalent in nature.
- **NaCN**: Sodium cyanide, which is predominantly ionic.

### Step 2: Reaction with AgCN
- When RBr is treated with AgCN in a highly polar solvent, the AgCN dissociates into Ag⁺ and CN⁻ ions.
- Since AgCN is covalent, the CN⁻ ion behaves as a nucleophile. However, due to the nature of AgCN, the nucleophilic attack occurs primarily through the nitrogen atom of the cyanide ion.
- The nitrogen atom has a lone pair of electrons that can form a bond with the partially positive carbon atom of RBr, leading to the formation of an isocyanide (RNC).

**Reaction:**
\[ RBr + AgCN \rightarrow RNC + AgBr \]

### Step 3: Reaction with NaCN
- When RBr is treated with NaCN, the NaCN dissociates into Na⁺ and CN⁻ ions.
- NaCN is ionic, and the CN⁻ ion can act as a nucleophile. However, in this case, the nucleophilic attack occurs predominantly through the carbon atom of the cyanide ion.
- This is because the carbon-carbon bond is stronger than the carbon-nitrogen bond, making it more favorable for the nucleophile to attack the carbon atom.

**Reaction:**
\[ RBr + NaCN \rightarrow RCN + NaBr \]

### Conclusion
- The key difference in products arises from the nature of the cyanide used:
- **AgCN** leads to the formation of isocyanides (RNC) due to the covalent nature of AgCN, allowing the nitrogen atom to participate in the reaction.
- **NaCN** leads to the formation of nitriles (RCN) because the ionic nature of NaCN favors nucleophilic attack at the carbon atom.